The cap gain is only paid on the profit made when selling. RSUs are taxed as regular income when they are awarded. Most likely the employees are high rate tax payers so it comes out at 40-45% tax + National Insurance.
With a HMRC approved scheme CGT effectively goes away and you only pay CGT after your yearly allowance and only on a real gain - no massive tax bill on underwater share options.
I don't know if this is a US-only arrangement, or if it's used in other countries as well.
You are correct about share options, on which your gain is only the price difference for which you pay CGT (or not). But the liquid tech companies (Google, FB, Twitter) give out direct stock which is taxed on their Fair Market Value at the time of vesting. RSUs are taxed the same way both in UK and US.